1. Field of the Invention
The present invention relates to vacuum deposition of thin films and molecular beam epitaxial (MBE) processing. In particular, the invention relates to the vapor effusion cell crucible which is the source of the molecular beam.
2. Description of Prior Art
A vapor effusion cell typically consists of a crucible containing a molecular source material, a heater to raise the crucible temperature, and a temperature sensor to measure the crucible temperature. In vacuum deposition of thin films, a vapor effusion cell is used to generate a molecular beam which condenses as a thin film on a substrate. A quantity of source material is placed in the crucible of an effusion cell and heated in vacuum to a temperature at which the material has sufficient vapor pressure for evaporation, so that a molecular flux emanates as a directed beam from the crucible. The substrate intercepts the beam such that the molecules of the beam condense on the substrate to form a thin film of the material.
For MBE film deposition, vapor effusion cells with precisely controlled molecular flux are required, particularly if several molecular beams are directed simultaneously at a common substrate, as in the deposition of the compound semiconductor gallium arsenide from separate gallium and arsenic effusion cells. In most effusion cell designs, the flux from the effusion cell is controlled by precise adjustment of the temperature of the source material, and means are provided for heating and measuring the temperature of an inert crucible containing the source material. For example, U.S. Pat. No. 4,239,955 describes a simple effusion cell for MBE which uses a thermocouple placed in a dimple in the base of the crucible to measure the crucible temperature. A more complex effusion cell which incorporates both temperature adjustment of the crucible and mechanical adjustment of the crucible orifice to control the molecular flux from the cell is disclosed in U.S. Pat. No. 5,714,008. An effusion cell with a thermocouple temperature sensor assembly heated by the thermal radiation from the crucible and the crucible heater is disclosed in U.S. Pat. No. 4,426,569. An effusion cell which uses an optical sensor to measure crucible temperature is disclosed in U.S. Pat. No. 5,540,780. More effusion cell designs are described in the following references:
S. H. Norrman, T. G. Andersson, S. P. Svensson and K. E. Flemming, "Highly Stabilised Evaporation Sources in a Water-Cooled Carousel Housing", J. Phys. E. Sci. Instrum., 15(7), pp.731-735 (1982) R. F. C. Farrow and G. M. Williams, "A High Temperature High Purity Source for Metal Beam Epitaxy", Thin Solid Films, 55, pp. 303-315 (1978)
R. A. Kubiak, P. Driscoll, and E. H. C. Parker, "A Simple Source Cell Design for MBE", J. Vac. Sci. Technol. 20(2), pp. 252-253 (1982)
Effusion cells with temperature sensors are calibrated by measuring the molecular flux from the crucible as a function of crucible temperature. For applications other than MBE deposition, such as the deposition of thin metal films used in the optics industry, precise control of molecular flux is often not required for satisfactory film deposition. Accordingly, for these applications, simple effusion cells are used which do not provide the means to measure the crucible temperature, and therefore cannot be conveniently calibrated. Typically, the deposited film thickness is monitored by a microbalance placed near the substrate, and deposition is terminated when the required film thickness is obtained. These effusion cells are usually called evaporation sources, and have the advantages of low cost and simplicity. Typical commercial evaporation sources are described in the Vacuum Evaporation Sources Catalog of the R. D. Mathis Company, P.O. Box 92916, Long Beach, Calif., 90809-2916. An example of an evaporation source from the R. D. Mathis Company consists of a model C9 molybdenum crucible installed in a model CH-10 crucible heater. The CH-10 is an electrical resistance heater with a tantalum sheet heating element and niobium radiation shields.
It is an object of the present invention to provide a low cost crucible with a thermocouple temperature sensor, which can be installed in a low cost commercial crucible heater such as the CH-10 crucible heater, so that a temperature controlled effusion cell can be realized at a cost comparable to that of an evaporation source without a crucible temperature sensor.
It is still another object of the present invention to provide a crucible which incorporates a simple means to secure a thermocouple temperature sensor inside a well (i.e. deep hole) within the base of the crucible, such that the thermocouple junction is substantially enclosed by the crucible bulk, but not in direct contact with the crucible. Thus, in a vacuum, heat transfer to the thermocouple is predominately by radiation, and the thermocouple temperature is slightly lower than, but reproducibly related to, the crucible temperature.